This application is to renew the Multicenter AIDS Cohort Study (MACS), a comprehensive quality-assured prospective cohort study of over 7000 men, which has contributed almost 1300 research papers on the natural history of HIV infection and the impact of effective combination antiretroviral therapies (cART). The follow-up of the existing cohort with continuing dynamic enrollment will enable elucidation of long-term effects of cART, including newer and evolving cART regimens, in men who have sex with men (MSM), the largest population impacted by HIV in the U.S. The overarching goals of the MACS are to continue to elucidate the biology of infection and the clinical outcomes of treated and untreated infection, combining immunologic, virologic, genetic and psychosocial approaches. The scientific agenda of the MACS focuses on four primary areas: 1) clinical epidemiology of HIV-related and non-AIDS-defining outcomes with particular emphasis on the effects of cART and age; 2) pathogenic mechanisms underlying HIV susceptibility, disease progression and treatment response including genetic and other molecular factors; 3) psychosocial factors, including substance use which influence risk of HIV infection, co-morbidities and treatment utilization; and 4) development of novel applied methodology. Following HIV-uninfected MSM with the same standardized protocols will facilitate distinguishing the contributions of HIV, age, genetics,co-infections and behaviors on the development of non-AIDS-defining outcomes including malignancies, and aging. The role of unresolved inflammation and immune activation, and more rapid senescence of the immune system, on disease course in treated HIV will be investigated. Data with linked specimens, particularly from over 700 HIV seroconverters, provides an unparalleled resource for studying the entire natural history of HIV, ranging from pre-infection (e.g. susceptibility) to infection, through treatment to old age and death. The MACS is and will continue to be ideally positioned to address these issues because of its long-term standardized follow-up, an extensive repository of over 1 million specimens that have been collected semiannually since 1984, an appropriate control group of HIV- MSM, and a seasoned and expanding cadre of experienced investigators.